This invention relates generally to the purification, e.g., sweetening, of natural gas.
Stringent specifications are typically applied to LNG (Liquified Natural Gas) processing. For example, carbon dioxide and often hydrogen sulfide are typically present in field gases and have to be removed to specific levels, e.g., less than 50 ppmv CO2 and less than 4 ppmv H2S) such as due to concerns related to processability, toxicity and corrosion, for example. More particularly, LNG processing typically requires cooling of natural gas to −160° C. This in turn requires that the CO2 concentration in the natural gas be less than 50 ppmv to avoid icing within the system.
Current state of the art processing to achieve such CO2 concentration levels in natural gas is by physical or chemical absorption in a column based process. Alkanolamine absorption process is a current state of art technology for this purpose. These amines are used as aqueous solvents to selectively absorb H2S and CO2 from sour natural gas streams. The sour gas is introduced at the bottom of an absorber and flows up the tower countercurrent to an aqueous amine stream. Within the tower, the acid gases are absorbed by the amine. The amine is described as being lean in acid gas as it enters the top of the absorber, and rich as it exits the bottom, loaded with acid gas. From the absorber, the rich amine is directed to the top of a stripping tower where a drop in pressure and application of heat enables the solvent to be stripped of the acid gases. The amine, again lean, is circulated back to the absorber for sweetening.
Offshore gas is often dehydrated on the offshore platform. Onshore gas is typically dehydrated a number of times between the well head and the consumer. Triethylene glycol (TEG) dehydrators are commonly the workhorse of the dehydration industry with only sporadic use of molecular sieves or calcium chloride. TEG systems typically circulate water-lean TEG counter-currently to the wet gas to absorb water vapor from the gas. The rich solvent is then heated in a regeneration unit to boil away the water. The lean solvent is returned to the absorber to complete the cycle. The absorber vessels are large towers or columns, 3-5 m in diameter, 10-30 m in height and weighing 50-100 tonnes, depending on throughput. The vessels also have special internals, including trays, random packing, and structured packing, to provide intimate contact between the gas and the liquid.
Conventional absorption towers in both amine sweetening systems and TEG dehydration systems are typically large in size, such as to require a large processing footprint, and heavy in weight. Further, they generally pose operational challenges such as liquid channeling, flooding, entrainment, and foaming.
These and other challenges are often accentuated in a floating LNG application such as may arise in offshore processing such on a ship, barge or platform, and such as may introduce motion parameters to the processing scheme. For example, such applications may suffer from motion-induced inefficiencies and process instability due to maldistribution of solvent in the column.
Thus there is a need and a demand for a processing technique and arrangement that minimizes or overcomes one or more of the process limitations typically associated with the sweetening of natural gas, particularly in the context of offshore applications.